In order to reduce the decline in cognitive abilities in the aging population, this proposal will examine the effect of aerobic fitness training on human cognition, brain structure, and the brain function of older adults. The main specific aims of this proposal include: 1) To test the hypothesis that improvements in aerobic fitness of older adults will lead to improved performance on a variety of cognitive processes, especially those supported by the frontal regions of the brain, 2) To test the hypothesis that the improvements in cognitive processes engendered by enhanced aerobic fitness will be supported by changes in underlying neural circuits, as inferred from patterns of fMRI activation, 3) To test the hypothesis that improvements in aerobic fitness, over the course of a 1 year intervention, will result in increases in gray and white matter volume in regions of the human brain (i.e. primarily frontal, medial temporal and parietal regions), 4) To test the hypothesis that increased levels of insulin-like growth factor 1 (IGF1) and decreases in markers of inflammation (i.e. C-reactive protein and interleukin-6) engendered by improvements in aerobic fitness will moderate the fitness benefits observed on measures of cognition, brain function and structure.